Contact Formation of Ag/Al Screen-Printing Pastes to Heavily B-Doped c-Si

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Contact Formation of Ag/Al Screen-Printing Pastes to Heavily B-Doped c-Si

Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften

(Dr. rer. nat)

vorgelegt von Susanne Fritz

an der

Mathematisch-Naturwissenschaftliche Sektion Fachbereich Physik

Tag der mündlichen Prüfung: 01.07.2016 1. Referent: Apl. Prof. Dr. Giso Hahn

2. Referent: Prof. Gerd Ganteför

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Abbreviations and Symbols

A^∗ eﬀective Richardson constant

Ac contact area of screen-printed contact ALD atomic layer deposition

Al₂O₃ aluminium oxide BBr₃ boron tribromide B₂O₃ boron trioxide BRL boron rich layer BSF back surface ﬁeld BSG borosilicate glass

qχS semiconductor electron aﬃnity

¯c constant in Varahramyan model CV capacitance-voltage measurement

Cz Czochralski

d ﬁnger distance

de emitter depth

df smallest ﬁnger distance for TLM

0 vacuum permittivity

r relative permittivity/ dielectric constant E00 characteristic energy

ECV electrochemical capacitance-voltage measurement

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EF n quasi-Fermi energy for electrons EF p quasi-Fermi energy for holes

Eg bandgap energy

Φ_B height of Schottky barrier Φ_M metal work function

φM potential corresponding to metal work function Φ_S semiconductor work function

φS potential corresponding to semiconductor work function

FE ﬁeld emission

F F ﬁll factor

F Fid ideal ﬁll factor

FIB focused ion beam

FM metallization fraction

η conversion eﬃciency

HCl hydrochloric acid

HF hydroﬂuoric acid

HNO₃ nitric acid

H2O2 hydrogen peroxide

HRTEM high resolution transmission electron microscopy

j current density

j0 dark saturation current density of ideal diode

j01 saturation current density of ﬁrst diode in two-diode-model j02 saturation current density of second diode in two-diode-model

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j0e emitter saturation current density

j0e,pass emitter saturation current density of passivated emitter j0e,met emitter saturation current density below metal contacts jmpp current density at maximum power point

jph photocurrent density

jsc short circuit current density

k Boltzmann constant

LIP light induced plating LT transfer length

mpp maximum power point

m^∗_t tunneling eﬀective mass

n ideality factor in one-diode-model

n1 ideality factor of ﬁrst diode in two-diode-model n2 ideality factor of second diode in two-diode-model

N doping density

NB boron concentration

NB,s boron surface concentration

O₂ molecular oxygen

pF F pseudo ﬁll factor

pin power density of incident light

PECVD plasma enhanced chemical vapour deposition pmpp power density at maximum power point POCl₃ phosphorous oxychloride

q elementary charge

ρc speciﬁc contact resistance

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RC contact resistance

Rem emitter contribution to Rt

RP shunt resistance RS series resistance

Rsh emitter sheet resistance

Rt total resistance measured by TLM SCR space charge region

SEM scanning electron microscopy SiN_x:H hydrogen rich silicon nitride SiO_x silicon oxide

SRH Shockley-Read-Hall

STEM scanning transmission electron microscopy

T temperature

TE thermionic emission

TFE thermionic ﬁeld emission

TEM transmission electron microscopy TLM transfer length method

V voltage

Vbi build-in voltage at Schottky barrier Vmpp voltage at maximum power point voc normalized open circuit voltage Voc open circuit voltage

W length of screen-printed contact WB depletion width of Schottky barrier wf ﬁnger width of screen-printed contact

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Introduction

With a market share exceeding 80% screen-printing is the metallization technique dominating in silicon photovoltaic industry. Although the share of other methods is likely to increase in the future, predictions see screen-printing dominating the market until 2025 [1]. The prevailing role of screen-printing is caused by the simplicity of the technique and the high throughput rate, making it ideal for industrial implementation.

For the metallization of the phosphorous emitter of conventional p-type solar cells commonly silver screen-printing pastes are used. The behaviour of these pastes has been thoroughly investigated in the last years and models for contact formation and current transport were proposed. Commonly silver crystallites that grow into the silicon below screen-printed contacts are considered as a prerequisite for low speciﬁc contact resistances that are required for high solar cell eﬃciencies.

For the metallization of boron emitters on n-type solar cells, however, these silver screen- printing pastes had shown to be inappropriate [2]. The high speciﬁc contact resistances published in literature can be attributed to the absence or reduced density of silver crystallites at the silicon surface.

In 2005 Kopecek et al. reported that the speciﬁc contact resistance of silver screen- printing pastes can be reduced to reasonably low values by the addition of a small amount of aluminium to the paste [2]. Several investigations followed regarding the de- velopment of applicable paste compositions [3, 4]. Today, boron emitters are commonly contacted with aluminium-containing silver pastes, reaching speciﬁc contact resistances

<4 mΩcm². However, the understanding of the contact formation process and the knowledge about the impact of diﬀerent factors, like ,e.g. wafer properties, on the contact is still incomplete.

Furthermore, with the addition of aluminium to silver pastes a new challenge came up. The low contact resistances are facilitated by the existence of deep metal spikes, penetrating the emitter. With a depth of usually >1μm these contact spots are deep enough to reach the space charge region and even the base. This behaviour is assumed to be responsible for losses in the open circuit voltage of ≈30 mV observed for cells metallized with silver-aluminium pastes [5–7]. Beside the contact formation process, also the exact origin of the high losses in open circuit voltage (Voc) and the impact of the deep contact spots on solar cell characteristics is only partly understood.

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The aim of this work is to develop a fundamental understanding of aluminium-containing silver screen-printed contacts on boron emitters. The main focus is on the examination of the mechanisms taking place during the contact formation of the printed paste with the silicon substrate in a ﬁring step and the parameters inﬂuencing these processes.

Therefore, detailed microstructural investigations are carried out, mainly by means of scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. In addition, the impact of screen-printed silver-aluminium contacts on cell characteristics of n-type solar cells shall be examined. These insights are aimed to support the devel- opment of new screen-printing pastes especially suited for boron emitters.

This work is divided into four chapters:

In the first chapter, the working principle of a solar cell is summarized and the basic structures of a conventional p-type silicon solar cell as well as a bifacial n-type cell are briefly discussed. In the following, two processes that had been challenging for the production of n-type solar cells are considered in more detail, the formation of the boron emitter and the surface passivation of this p⁺ doped layer. Screen-printing is then introduced as metallization technique for solar cells and the requirements for screen-printing pastes are summarized, followed by a short overview of alternative metallization methods. The next section is dealing with the IV-characteristics of solar cells, introducing relevant parameters. Subsequently, losses in solar cells are discussed, fo- cusing on the impact of the metallization. In the last section of the first chapter, the metal-semiconductor contact and the current transport over the interface is considered.

The second chapter brieﬂy introduces the main characterization methods used in this work. First the electrical characterization of the solar cells, the contact resistance and the emitter proﬁle are shortly discussed. In the following, the microstructural characterization methods SEM, EDX and transmission electron microscopy (TEM) as well as the sample preparation procedures are presented.

The main part of this work, chapter three, deals with the contact formation of silver- aluminium screen-printing pastes to boron emitters. First existing publications ex- amining screen-printed silver contacts to phosphorous emitters and silver-aluminium contacts to boron emitters are summarized. Hereafter, the standard experimental pro- cedure, applied in most experiments in this work is introduced. In the following sections, single factors of the paste-wafer systems are varied to investigate their impact on the contact formation process separately. According to this method, detailed microstructural analyses are carried out in different experiments. First the role of different paste components is analyzed. Then the influence of emitter properties on contact formation and specific contact resistance is studied and the experimental results are compared

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Introduction

with analytical calculations of the contact resistance according to the Schottky model.

In the following sections, the effect of surface properties like morphology and crystal orientation of the silicon base material, as well as the role of the dielectric layer between screen-printed contact and wafer surface is evaluated. For a deeper understanding of the processes occurring during contact firing, the influence of the peak firing temperature on the contact formation is analyzed. The microstructural study is then completed by a TEM analysis of contact spots. In the last section of chapter three, the results of the different sections are summarized in a model for the contact formation of aluminium- containing screen-printing pastes to boron emitters.

In the last chapter the understanding of silver-aluminium screen-printed contacts is deepened by investigating the influence of the metallization on the IV-characteristics of bifacial n-type solar cells. Especially, the influence of the aluminium and glass content in the paste on losses in the open circuit voltage as well as the fill factor are regarded and the origin of these losses is evaluated.

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1 Silicon Solar Cells

1.1 Working principle

The operation principle of a solar cell, the direct conversion of light into electrical energy, is based on the internal photoelectric effect. By the absorption of an incident photon with an energy larger than the band gap energy (Eg = 1.1 eV for silicon), an electron is excited from the valence band into the conduction band of the semiconductor - an electron-hole pair is generated. The spatial separation of the charge carriers is facilitated by the electric field generated by a doping gradient close to the wafer surface: By the integration of foreign atoms into the silicon lattice an excess concentration of electrons (n-doping) or holes (p-doping) is realized. The concentration gradients at the transition of n- to p-doped silicon at the pn-junction are balanced by the diffusion of electrons from the n-doped to the p-doped region and the reverse diffusion of holes. The remaining ionized lattice atoms form a space charge region (SCR). The generated electric field induces a drift current that is opposed to the diffusion current.

In thermal equilibrium drift and diﬀusion currents balance. The balance of the Fermi levels in the semiconductor in thermal equilibrium leads to a band bending at the SCR.

Under illumination, the semiconductor is no longer in thermal equilibrium and the distribution of charge carriers can no longer be described using the equilibrium Fermi energyEF. However, the individual population of electrons in the conduction band and holes in the valence band can be in equilibrium, and individual Fermi niveaus, called quasi-Fermi levels, can be applied [8]. The diﬀerence between the quasi-Fermi levels for electronsEF nand holesEF p determines the open circuit voltageVoc of the solar cell.

Conventional silicon solar cells, which present the majority of industrially produced silicon solar cells, consist of a lowly doped, thick p-type base and a thin n-type emitter, which is commonly heavily phosphorous doped. In ﬁgure 1.1 the band diagram of such a silicon solar cell is shown.

The course of the potential in the SCR pushes electrons, that are generated in the p-doped base and reach the SCR, into the emitter. The electrode contacting the emitter shows the characteristics of a rectifying metal-semiconductor contact, a so-called

”Schottky-contact”. The characteristics of this type of contact will be discussed in

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metal p-Typ Si energy

valence band

E

E_Fn

E_Fp

electron

V_OC

BSF

E

p⁺

SCR

metal hole

hQ

n-type Si conduction band

Figure 1.1: Schematic presentation of the band diagram of a p-type solar cell under illumination after [9, 10]. The dimensions of the diﬀerent regions are not to scale.

section 1.7. At the back side of the solar cell a highly p-doped (p⁺) region exists. The resulting band bending repels electrons from the back side electrode that shows an ohmic voltage-current behaviour. The generated electric ﬁeld is called back surface ﬁeld (BSF).

In the following sections, the basic structure of a conventional p-type solar cell (p-type base material) and solar cells with n-type base material are presented.

1.2 Conventional p-type solar cell

In ﬁgure 1.2 the schematic structure of a conventional p-type silicon solar cell is shown.

The p-doped base material typically has a thickness of 150-200μm. The highly n- doped (n⁺) emitter on the front side of the cell, commonly formed in a phosphorous diﬀusion, features a high dopant surface concentration and has a thickness of some 100 nm. To reduce reﬂection of the incident light the front side of the cell is textured.

For mono-crystalline base material a structure of random pyramids is therefore applied.

A hydrogen-rich silicon nitride layer (SiN_x:H) serves as anti-reflection and passivation layer (compare section 1.3.2). The front side of the solar cell is contacted with a grid of thin screen-printed silver fingers that penetrate the dielectric layer. The back side electrode is formed by a full-area screen-printed aluminium layer that alloys with the silicon during contact firing forming an aluminium doped p⁺ region [11]. As aluminium is not solderable, a silver-aluminium paste is locally printed on the silicon to facilitate soldering the back side for module integration.

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1.3 Solar cells with n-type base material

base (p)

emitter (n⁺)

p⁺

dielectric layer

Al contact Ag fingers

textured surface

Figure 1.2: Schematic structure of a conventional p-type silicon solar cell with full area aluminium back contact.

1.3 Solar cells with n-type base material

The predominant role of p-type base material used for the production of crystalline silicon solar cells is based on historical reasons as well as technological considerations.

In the early stage of photovoltaics cell production energy generation from solar cells was expensive. Therefore, solar cells were mainly used for space application. p-type material proved to be signiﬁcantly more resistant to high energy space radiation than n-type material [12]. As a consequence, research concentrated on solar cells produced from boron or gallium doped p-type material. The onset of terrestrial application of solar cells was later based on the achievements of space industry and as a result p-type silicon was adopted as the standard material for terrestrial silicon solar cells.

Additionally, technological challenges prevented the rise of n-type technology. Two important issues will be discussed in following sections, namely the formation of p⁺-doped boron emitters and the passivation of these heavily doped layers. Another challenge, the metallization of boron emitters is topic of this work and will be discussed thoroughly in the chapters 3 and 4.

Despite historical and technological obstacles, in the past decade the interest in n-type base material has grown as it exhibits some advantages over p-type silicon considering terrestrial application.

For solar cells based on boron doped p-type material the degradation of cell eﬃciency under illumination, known as light induced degradation, is a serious issue [13]. As this degradation is based on an increased recombination due to a metastable boron-oxygen defect, n-type material does not suﬀer from it [14]. In 2006, however, Herguth et al.

discovered the so called regeneration process in which the initial cell eﬃciency can be permanently recovered [15]. Since then, a lot of research was done in this ﬁeld. In 2014,

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Wilking et al. presented a high speed regeneration process that can be completed in

<10 s, making the in-line implementation of regeneration in the solar cell production feasible [16].

Even if light induced degradation seems to be no longer a serious issue for solar cells based on boron doped silicon - at least in the long run - n-type silicon shows superior material properties as the minority carrier lifetime of n-type material is less sensitive to common metal impurities than p-type silicon due to asymmetric capture cross-sections for electrons and holes [17, 18].

In the last years several n-type cell concepts have emerged. The cell concept most closely resembling the conventional p-type cell introduced in the previous section is the aluminium rear emitter n-type cell (”PhosTop” cell) [19, 20]. Principally, the process sequence of the conventional p-type cell can be adopted, making this cell type interest- ing for industrial application. As the emitter is situated at the rear side of the solar cell, base material with a high minority carrier diﬀusion length is necessary, to enable the generated charge carriers to reach the SCR. Therefore, only high quality base material can be used. With other n-type cell concepts like interdigitated back contact (IBC) [21]

and heterojunction with intrinsic thin layer (HIT) [22] very high cell eﬃciencies (>24 %) can be reached. However, these concepts are very complex and require a large number of process steps. A detailed summary of n-type cell concepts can be found in [23].

In this work, a bifacial n-type cell concept [5, 24, 25] is applied to characterize the metallization induced losses of silver-aluminium screen-printed contacts. The advantage of the bifacial cell design is that light can be collected on both sides of the solar cell and thereby contribute to the current generation. The n⁺-side design is similar to the one applied on conventional p-type solar cells. To allow light to enter from the back side of the cell the full area aluminium contact applied on conventional p-type solar cells or aluminium rear emitter n-type cells is replaced by a ﬁnger grid design similar to the one on the front side. In principle bifacial cell concepts are feasible for n- and p-type base material. To beneﬁt from the bifacial design, the material used must be of high quality, as charge carriers generated at the non-emitter side of the cell must reach the SCR to contribute to power production.

In ﬁgure 1.3 a schematic of the basic structure of a bifacial n-type solar cell is shown.

The cell features a p⁺-emitter, which is generally formed in a boron diffusion, as will be discussed in the following section 1.3.1. The n⁺-doped layer, generating a BSF, is formed in a POCl₃-diffusion. On both sides of the cell a surface texture is applied to reduce reflection. For additional anti-reflection and passivation purposes, a dielectric layer is deposited. The metallization is realized by screen-printing a finger grid on each

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base (n)

emitter (p⁺)

n⁺

dielectric layer

dielectric layer Ag/Al fingers

Ag fingers

textured surface

Figure 1.3: Schematic structure of a bifacial n-type solar cell.

side. To contact the n⁺-region a standard silver screen-printing paste is used, for the metallization of the boron emitter a silver-aluminium paste is applied.

Despite the discussed advantages of n-type material and the high potential of the material proved with several cell concepts, the market share of silicon solar cells based on n-type silicon was only around 5 % in 2014 [1]. There are several reasons for the slow growth of the n-type solar cell market. Next to the necessity of new fabrication processes and equipment, the high price of n-type wafers compared to p-type material increases the total costs of n-type solar cells. Additionally, for solar cells with rear junction design, like the PhosTop and IBC cells, base material of very high quality is necessary, as discussed before. Additionally, most cell concepts feature complex cell processes, what makes them less interessting for industrial implementation.

1.3.1 Boron emitter formation

Within this work the boron emitters were by default diﬀused in a boron tribromide (BBr₃)-based process. Other possibilities for the formation of boron emitters, applied at the University of Konstanz, are the diﬀusion of boron from boron doped silicon oxides (SiO_x:B) deposited with plasma enhanced chemical vapor deposition (PECVD) [26, 27]

or atmospheric pressure chemical vapor deposition (APCVD) [28]. Diﬀusion sources like boron trichloride (BCl₃) [29], sources applied by screen-printing [30,31], ink-jet [32]

and spin-on [33] techniques as well as ion-implantation [34] are not used within this work and will therefore not be further discussed.

For the boron emitter diffusion from a liquid BBr₃ source wafers are positioned in an upright position in a glass boat and brought into a quartz tube diffusion furnace as can be seen in figure 1.4. During the deposition step, N₂ is directed through a bubbler

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containing BBr₃ where it acts as a carrier gas, transporting the BBr₃ into the diﬀusion tube that is heated to temperatures above 900°C. The additionally introduced O2 reacts with the BBr3 and produces boron trioxide (B2O3) according to

4 BBr3+ 3 O2 −→2 B2O3+ 6 Br2. (1.1) The B₂O₃ condenses on the silicon wafers. In a further reaction, B₂O₃ reacts with the silicon and forms SiO₂ as well as elementary boron, which can diﬀuse into the silicon bulk:

2 B2O3+ 3 Si−→4 B + 3 SiO2. (1.2) The SiO2 and the B2O3 are partly mixing forming a borosilicate glass (BSG).

In the next process step, the drive-in step, the elementary boron diffuses into the wafer according to the diffusion gradients. At the applied high temperatures, diffusion of boron in silicon takes place via interstitial and vacancy diffusion mechanisms [35]. In this process step a very high concentration of boron can occur at the silicon surface and react with the silicon according to the reaction

Si + 6 B −→SiB₆. (1.3)

This boron rich layer (BRL) that forms at the wafer surface can degrade the charge carrier lifetime due to enhanced Shockley-Read-Hall (SRH) recombination [36]. It cannot be removed in dilute hydroﬂuoric acid (HF) like the BSG. Diﬀerent ways to avoid the formation of the BRL or to oxidize the SiB₆ layer after formation and later remove it have been reported, making it possible to maintain high charge carrier lifetimes [37,38].

An alternative is the removal of the BRL in a mixture of nitric acid (HNO3), acetic acid (CH3COOH) and HF [39].

The emitter can be characterized by parameters like,e.g. dopant surface concentration

bubbler with BBr₃ N₂

O₂

N₂+ BBr₃

quartz tube glass boat

exhaust heating

wafer

Figure 1.4: Schematic of diﬀusion furnace for BBr₃ diﬀusion. The wafers are positioned in a glass boat. BBr₃ from the bubbler is transported into the quartz tube by the carrier gas N₂. O₂ reaches the tube via an extra supply.

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Ns, the emitter depth de, the sheet resistance Rsh, the overall emitter profile and its lateral homogeneity. To quantify recombination in the emitter, the emitter saturation current densityj0eis used (compare section 1.6.2). These characteristics can be adjusted by diffusion parameters like temperature, gas flows, and drive in time. A systematic investigation of different diffusion parameters on boron emitter characteristics can be found in [40].

1.3.2 Passivation of boron emitters

The surface of a semiconductor represents a region with a high density of crystal defects due to the abrupt end of the semiconductor and therefore is a region of high recombination activity [41]. In the case of silicon solar cells, a high surface recombination at the front surface results in a reduction of the open circuit voltageVoc. The surfaces are therefore passivated to reduce recombination.

The recombination activity of a surface can be quantiﬁed by means of the surface recombination velocity (SRV). In solar cell application the emitter saturation current density j0e is often used to indicate the quality of a surface passivation. However, j0e not only takes into account the surface recombination but all recombination mechanisms occurring in the highly doped region.

There are two types of surface passivation: By chemical passivation ”dangling” bonds of the silicon lattice are saturated and thereby the density of defect states is reduced.

Field effect passivation is based on fixed charges positioned in the passivation layer that build an electric field which rejects one type of charge carriers (e.g., minority carriers) and attracts the other type [42]. Generally, dielectric passivation layers for the passivation of n⁺-surfaces like SiN_x:H or silicon oxide (SiO_x) rely on both mechanisms:

dangling bonds are saturated and ﬁxed positive charges in the dielectric layer prevent holes from reaching the interface.

For p⁺ surfaces, dielectric layers used for the passivation of n⁺ layers do not yield com- parably low surface recombination velocities [43, 44]. Especially the passivation quality of SiN_x:H layers shows to be insuﬃcient. One explanation are the built-in positive charges commonly present in the silicon nitride. attracting electrons to the interface that build an inversion layer. This leads to an increased recombination activity [45].

Altermatt et al. suggested two additional mechanisms responsible for the low passivation quality [43]. First, the capture cross sections of defects at the silicon-silicon nitride interface show diﬀerent values for holes and electrons [46]. As there are defects with considerable higher values for holes, they propose that the density of defects is independent of dopant type, but that these defects cause more recombination in p-type

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than in n-type layers. Second, boron related defects could increase the total defect density. The passivation of p⁺ layers with SiO_x shows the same problems [47] but less pronounced as the density of ﬁxed charges is smaller compared to SiN_x:H [48]. Further- more the passivation quality of silicon oxide degrades over time but can be restored by a thermal anneal [43, 49]. Despite these problems, moderate to low emitter saturation current densities can be realized. An overview of measured values for the passivation with thermal oxide is given in [50]. Mihailetchiet al. achieved very good values as low as 23 fA/cm² for a passivation stack consisting of a thin wet chemical oxide and silicon nitride for a 60 Ω/boron emitter on a planar surface [51].

To enable passivation of a p-doped surface not only by means of chemical passivation but also by ﬁeld eﬀect passivation, a dielectric with negative built-in charges, that re- pel the minority carriers in the p⁺ region (the electrons) from the surface, is needed.

Amorphous aluminum oxide (Al₂O₃) is well suited for this purpose and gained in importance in recent years. The aluminium oxide layer can be deposited by atomic layer deposition (ALD). The passivation needs to be ”activated” by annealing the wafers at around 400°C in a N₂ ambient [52]. Richteret al. attribute the improvement of passivation quality in the activation step to the hydrogenation of the silicon-dielectric interface by hydrogen originating from the Al₂O₃-layer [53]. Hoex et al. reported of excellent emitter saturation current densities of ≈ 10 fA/cm² for 54 Ω/ and ≈ 30 fA/cm² for

>100 Ω/BBr₃-based emitters passivated with 30 nm ALD Al₂O₃ after anneal [54]. As the ALD method is time and cost intensive, its industrial implementation is diﬃcult.

Recently, it has been shown, that good passivation qualities can be achieved with aluminium oxide deposited by PECVD [55] and APCVD [56].

Due to a small refractive index (below 1.7 for relevant wavelengths [57]), aluminum oxide cannot serve as a single layer anti-reflection coating. To enable both, good elec- tronic and optical properties, aluminium oxide layers are generally capped by a silicon nitride layer for solar cell application. This allows the application of thinner aluminium oxide layers. An additional anneal step is not necessary in this case, as the passivation effect of the aluminium oxide/silicon nitride stack is also activated in the firing process at the end of solar cell production [58].

1.4 Metallization of silicon solar cells

In this work, screen-printing metallization for boron emitters is investigated. Therefore, boron emitters were exclusively contacted with this method. In the following the screen- printing technique is introduced and an overview of other metallization methods is given.

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1.4 Metallization of silicon solar cells

1.4.1 Screen-printing metallization

Screen-printing is the most common technique for the metallization of silicon solar cells [59] and, in the opinion of experts in the ﬁeld, will be in at least the next 10 years [60]. The reason for its dominant role can be found in high throughputs achiev- able in industry and the relatively simple, cost eﬀective processes.

In a basic screen-printing process a metallization paste is pressed by a squeegee through openings in a screen and deposited on a wafer. For the application in solar cell production, the screen is usually based on a wire net, consisting of stainless steel or polyester.

The screen is covered with a photosensitive emulsion that allows the deﬁnition of the required pattern by photolithography, facilitating the printing of very thin lines. Today line widths of 35μm on lab scale and <50μm in industry can be achieved [59, 61].

Single print techniques are the most frequently used methods, as only one printing step is applied. A possibility to improve contacts applied by screen-printing or other printing methods are double and dual print [61, 62], that allow optimized ﬁnger and busbar properties.

Screen-printing pastes for the front side metallization of solar cells need to meet several requirements. The ﬁrst requirements are related to the screen-printing process. Pastes must be well printable but at the same time allow a high height-to-width aspect ratio to facilitate a high lateral conductivity while minimizing shadowing. To meet both demands, the rheology of the paste is adapted to the process. Pastes for screen-printing show a pseudoplastic behaviour. That means, the viscosity of the paste is reduced when the paste is exposed to strong shear forces, as it is the case when pressed through the screen, and increases rapidly when the screen is removed from the substrate. This prevents the dispersion of the paste and at the same time assures that the paste is ﬂuid enough to pass the thin openings in the screen. Additionally, the adhesion of the paste to the substrate must be high enough to avoid the removal of the printed contacts with the screen.

The second group of requirements concerns the properties of the ﬁnal contact. The contact must feature a high lateral conductivity and a low contact resistance to the substrate. Additionally a good mechanical adhesion and long term stability of the contacts must be guaranteed.

To fulﬁll these demands, screen-printing pastes for the front-side metallization of standard solar cells with phosphorous emitters contain three main constituents to fulﬁll these requirements:

• organic components (binder and solvent)

• up to 5 % glass fritt

• up to 90 % silver powder

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The organic components are responsible for the printing properties and prevent the drying-out of the paste. They are burned out in the drying and ﬁring process after paste deposition. The commonly used binder-solvent system consists of ethyl cellulose and terpineol.

The glass fritt fulfills different roles. It affects the wetting of the wafer with the paste and therefore the mechanical contact between paste and silicon [63]. Additionally, the sintering of the contact is influenced. The glass removes the anti-reflection layer on the wafer and establishes a direct contact between silicon and the silver contained in the paste. For contacting phosphorous emitters, the glass is further necessary to facilitate the growth of silver crystals as will be discussed in section 3.1. Typically a lead borosilicate glass is used.

Silver is used as the metal component in the paste due to its high conductivity. Next to the reduction of shadowing, the main motivation to reduce the ﬁnger width of front side contacts is to minimize silver consumption in order to save costs.

The back side of a conventional p-type solar cell is commonly contacted by full-area screen-printing of an aluminium paste on the wafer. During the ﬁring process, aluminium and silicon alloy. As aluminium is not solderable, silver-aluminium pads are commonly integrated into the rear side design of the solar cells to facilitate module integration. A detailed analysis of full-area screen-printed aluminium contacts is given in [64].

For contacting boron emitters with silver screen-printing pastes commonly around 5 % of aluminium are added to the paste, as silver screen-printing pastes usually used on phosphorous emitters do not result in reasonable contact resistances [2, 23]. The investigation of the contact formation of these aluminium-containing silver screen-printing pastes to boron emitters is the main topic of this work.

After screen-printing and drying, contacts are fired in a high temperature process. This is commonly done in an infrared (IR) conveyor belt furnace. During this firing step, the electrical contact between printed metal paste and emitter is formed. Existing models for the contact formation of screen-printing pastes in the firing step will be summarized in sections 3.1 and 3.2.

1.4.2 Other industrial relevant metallization methods

Even though screen-printing is the method commonly used in silicon photovoltaic industry for the metallization of solar cells, other techniques gain in importance. This process is driven by the limitations of screen-printing considering ﬁnger width as well as silver consumption and metallization related losses, including contact and line resistance, that limit cell eﬃciency.

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1.5 Current-voltage characteristics

Another technique for the deposition of thick ﬁlm pastes, that is assumed to be introduced in mass production in the next years, is stencil printing [1]. The advantage of stencil printing is the ability to print ﬁner lines with higher aspect ratio [65]. For stencil printing the same equipment and pastes as for screen-printing can be used, except of the screens that are made of metal foil.

Dispensing represents a contactless method to apply a thick film paste on a substrate [66]. Principally, thin fingers with high aspect ratio are feasible, but the dispersion of homogeneous thin lines through syringes is challenging. An advanced dispensing method is the coextrusion technology. Here the actual paste is compressed by a sacrificial paste that is also printed on the wafers but completely burned off during firing [67, 68]. Another non-contact deposition technique is ink jet printing [69]. How- ever, these methods are predicted to stay niche products in the next years [1].

A promising candidate for future metallization technology is light induced plating (LIP) [1, 60]. Besides direct plating of nickel/copper contacts [70], a seed layer can be deposited by other techniques like,e.g., screen- or stencil printing and thickened by LIP of copper or silver [71,72]. Plated contacts principally enable high cell performance, due to low contact and line resistances (compare section 1.6.4), and the utilization of lower cost metals. However, the direct LIP technique is limited on the metallization of n-type surfaces [73].

A detailed overview of industrial relevant metallization techniques as well as techniques rather limited to lab application up to now is given in [74].

1.5 Current-voltage characteristics

In principle a silicon solar cell is a large area diode. Without illumination it shows the same current voltage characteristics as a normal diode (see ﬁgure 1.5 dark IV- curve) that can be described by Shockley’s ideal diode equation [75]. Although in the following jV-curves are presented (current density versus voltage), they will be referred to as IV-curves, as commonly done. Under illumination the dark curve is shifted into the fourth quadrant by the photocurrent density jph generated by the solar cell. The sign of this shift is caused by the direction ofjph that ﬂows into the opposite direction as the forward biased diode. This behaviour is described by equation 1.4:

j =j0

exp

qV nkT

−1

−jph, (1.4)

whereV is the voltage drop across the solar cell, q the elementary charge,k the Boltz- mann constant and T the temperature in Kelvin. j0 represents the dark saturation current density of an ideal diode which is the sum of the saturation current densities of

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0 100 200 300 400 500 600 700 -40

-20 0 20 40

-40 -20 0 20 40

p(mW/cm 2)

j(mA/cm2 )

V (mV) j_mppэV_mpp = FF

j_scэV_oc

|jэV|

V_mpp

j_mpp j_sc

p_mpp

mpp V_oc dark jV-curve

illuminated jV-curve power density curve

Figure 1.5: Illuminated and dark IV-curve and the power density versus voltage of a solar cell, illustrating the IV-parameters.

base and emitter,j0b and j0e respectively:

j0 =j0b+j0e. (1.5)

Equation 1.4 accounts for radiative, Auger, Shockley-Read-Hall (SRH) and surface recombination occurring in the base and the emitter of the solar cell, leading to a diode ideality factor n = 1. To include SRH recombination in the SCR, values of n > 1 have to be assumed, or a second diode in parallel with the ﬁrst one with an ideality factor n2 has to be introduced. For recombination in the SCR via defects close to the middle of the band gap n2 = 2 [76]. If further the series resistance of the solar cell RS, accounting for ohmic losses, and a parasitic shunt resistance RP are included, a solar cells IV-characteristics can be described by the commonly used two-diode-model:

j =j01

exp

q(V −jRS) n1kT

−1

+j02

exp

q(V −jRS) n2kT

−1

+ V −jRS

RP −jph. (1.6) The first term in equation 1.6 describes the influence of emitter and base. The second term accounts for recombination in the space charge region. The influence of leakage currents is described by the third term. Figure 1.6 depicts the equivalent circuit diagram of a solar cell described by the two-diode-model.

In real solar cells both ideality factors depend on injection level and type of recombination and can, therefore, deviate from their ideal values n1 = 1 and n2 = 2 [77].

The most important parameter to describe a solar cell’s ability to convert solar energy into electrical energy is its eﬃciency η which is deﬁned as the ratio of the maximum power density that can be extracted out of the solar cell pmpp and the power density of

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1.5 Current-voltage characteristics

j₀₁ j₀₂ R_P

R_S

j_ph

V,j

Figure 1.6: Equivalent circuit diagram of solar cell according to the two-diode-model.

the incident lightpin. η can be calculated by:

η= pmpp

pin = jmpp·Vmpp

pin (1.7)

wherejmpp andVmppare the current density and voltage at the point of maximum power output (maximum power point (mpp), see ﬁgure 1.5).

Beside the point of maximum power output, two important points in an IV-curve of a solar cell are the open-circuit voltage Voc, that is deﬁned as the V-axis intercept of the IV-curve, and the short-circuit current density jsc, representing the j-axis intercept of the curve.

With the assumptions jph >> j0 and |jsc| = jph, the open circuit voltage Voc for an ideal diode can be calculated by equation 1.4:

Voc = kT q ln

jsc

j0

. (1.8)

Withjsc andVoc another important parameter, the ﬁll factorFF, is deﬁned by the ratio of the maximum power density (pmpp =jmppVmpp) and the product of jsc and Voc.

FF = jmppVmpp

jscVoc (1.9)

This is visualized in ﬁgure 1.5 by the ratio of the two rectangles.

The fill factor is significant considering losses associated with solar cell metallization, as RS and RP influence the IV-curve and therefore FF as will be discussed in section 1.6.4. Green [75] proposed an expression to determine an ideal fill factor FFid free of losses related to series resistance, shunt resistance and recombination that is only a function of Voc.

FFid = voc−ln(voc+ 0.72)

voc+ 1 (1.10)

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with the ideality factor n and the normalized voltage voc >10 voc= Voc

nkTq

. (1.11)

A third fill factor that is free of series resistance effects is the pseudo fill factor pFF that can be obtained by SunsVoc measurements (compare section 2.1.2).

The two-diode-model parameters of a solar cell (j01, j02,RP and RS) can be extracted from the IV-measurements by ﬁtting the two-diode-model to the measured curves.

For the different parameters different fitting windows are used. In figure 1.7 a semi- logarithmic plotted dark IV-curve and the approximate fitting windows for the different parameters are illustrated.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 1E-8

1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1

j₀₁ j₀₂

R_P j(A/cm2 )

voltage (V)

R_S

Figure 1.7:Dark IV-curve with approximate ﬁtting windows for parametersj₀₁,j₀₂,R_P and R_S.

1.6 Metallization induced losses in solar cells

One main goal of the research on silicon photovoltaics is to increase the eﬃciency of the solar cells. In order to achieve this, the loss mechanisms in the solar cells need to be analyzed. Losses in solar cells can be divided in optical and electrical losses, the latter can be either of recombinative or ohmic origin. In the following sections primarily losses induced by the metallization of the solar cells are discussed. Other loss mechanisms are only mentioned brieﬂy for completeness.

1.6.1 Optical losses

Optical losses lower the short-circuit current density jsc and thus the power generated by the solar cell. As a result of reﬂection at the silicon surface and the front side

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1.6 Metallization induced losses in solar cells

metallization, as well as transmission of light that is not absorbed in the solar cell, the generation of electron hole pairs in the cell is reduced. To lower the external reflection of a solar cell, the front side is textured reducing the reflection to around 10 %. Additionally, a dielectric layer serving as anti-reflection coating is deposited.

Optical losses induced by metallization are caused by the shading of the silicon surface by the front side contacts. Current industrial solar cells feature a contact grid with a typical finger width of 60-100μm and a finger distance of 1-2 mm. Including the busbars, 5-10 % of a solar cells front surface are covered by contacts. The front side shading can be lowered by reducing the area covered with metal. However, the reduction of finger width results in a higher series resistance, as will be discussed in section 1.6.4.

Therefore, ﬁngers are optimized to reach a high height-to-width aspect ratio. Optical losses are not investigated within this work and are further discussed in [74, 78].

1.6.2 Recombination losses in the emitter

Recombination losses in the emitter of a solar cell can be quantiﬁed by the emitter saturation current density j0e, which is a part of the saturation current density j01

(compare equation 1.5). For non-metallized samplesj0e depends on the emitter doping proﬁle, the surface passivation and on surface texture. With decreasing sheet resistance, recombination in the emitter region increases. If no dopant precipitates are present in the emitter, Auger recombination is dominating in the highly doped regions [79, 80].

For passivated emitters this leads to increasedj0e values. For non-passivated emitters, the inverse behaviour can be observed, due to an increasing inﬂuence of the surface recombination on j0e for higher sheet resistances [81].

Typical boron emitters used within this work feature emitter sheet resistances around 50 Ω/ and show j0e values of ≈ 50 fA/cm² on plane surfaces passivated by Al₂O₃. For non-metallized samples j0e can be determined by photoconductance decay lifetime measurements [82, 83].

For a solar cell with a contact grid, j0e is composed of the emitter saturation current density of the passivated emitter next to the contactsj0e,pass and the emitter saturation current density below the metal contacts j0e,met. In general,j0e,met > j0e,pass. Reasons for that are metal spikes growing into the emitter, a possible in-diﬀusion of metal impurities from the paste to the emitter [84, 85] and for screen-printing metallization the removal of the surface passivation below the metal grid by the glass frit in the paste.

For uniform j0e,met, the total emitter saturation current density can be calculated by j0e =j0e,pass(1−FM) +j0e,met·FM (1.12)

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with FM being the fraction of the metallized cell area. Diﬀerent methods to determine j0e,met based on equation 1.12 have been proposed [7, 86].

1.6.3 Recombination losses in the space charge region

SRH recombination in the space charge region is a further source for efficiency losses due to the front side metallization. In the two-diode-model this is accounted for by the saturation current density j02. j02 primarily influences F F. Its impact on Voc is less pronounced and the influence of j01 on Voc dominates. j02 is increased by screen- printing metallization [87]. This can,e.g. be due to the diffusion of metal into the SCR and recombination at the highly recombinative metal-semiconductor interface at deep metal spikes, that reach the SCR.

1.6.4 Resistance related losses

Resistance related losses in solar cells are incorporated in the two-diode-model by the inclusion of a series resistance RS and a shunt resistance RP term, as described in section 1.5.

Shunt resistance losses

The origin of low shunt resistances are parasitic leakage currents. These currents can ﬂow around the edges of a solar cell (incomplete edge isolation) or over local crystal defects in the space charge region like grain boundaries or impurities. Additionally, the metal of the contacts can be in direct contact with the base as a result of an imperfect emitter or deep metal spikes originating from the metallization that penetrate the emitter and reach the base.

In figure 1.8 (a) the influence of the shunt resistance RP on the solar cells IV-curve is illustrated. A reduced shunt resistance shifts the maximum power point and therefore primarily affects the fill factor. For very low RP the Voc is also reduced. jsc is not influenced by the shunt resistance.

Series resistance losses

Figure 1.8 (b) shows the inﬂuence of the series resistance RS on a solar cells IV-curve.

For increasing RS ﬁrst the maximum power point and thereby the FF is inﬂuenced.

For very high RS additionally the short circuit current is reduced.

As a rule of thumb, Pysch [88] proposed an approximation for the relationship between FF and RS: an increase of 1 Ωcm² reduces the ﬁll factor by ≈5 % absolute.

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1.6 Metallization induced losses in solar cells

0 -10 -20 -30 -40

0 100 200 300 400 500 600 700

V (mV)

j (mA/cm2) _R

P=10 :cm²

10000 5000

20

100 500

(a)

0 100 200 300 400 500 600 700 0

-10 -20 -30 -40

RS=20 :cm² V (mV)

1 00.5

10 5 2

j (mA/cm2)

(b)

Figure 1.8: Schematic illustration of effect ofR_P (a) andR_S (b) on IV-characteristics after [41]. The maximum power point and with thisFF are firstly influenced by lowR_P or enhanced RS values. (a) for lowRP Voc is reduced. (b) for highRS jsc is reduced.

In ﬁgure 1.9 the diﬀerent resistances that occur in a conventional solar cell and contribute to the total series resistance RS are schematically illustrated. RS is composed of:

R1: lateral resistance of back contact R2: contact resistance of back contact R3: base contribution to RS

R4: lateral resistance of the emitter

R5: contact resistance between metal ﬁnger and emitter R6: lateral resistance of metal ﬁnger (line resistance) R7: lateral resistance of busbar

For a conventional solar cell, a full area back contact is applied and therefore the contributions of R1 and R2 can be neglected.

For bifacial cells, resistances analog to the ones on the front side of the solar cell appear

Base Emitter

Back contact

Metal fingers Busbar

R₁ R₂ R₃

R₄ R₄

R₅ R₅

R₆ R₆

R₇

Figure 1.9: Schematic illustration of diﬀerent resistances contributing to R_S for a conventional solar cell. For a bifacial cell, the resistances network on the back side is analogue to the front side.

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at the rear side. In this case the resistance network of ﬁgure 1.9 can be mirrored at the base with R4 on the rear side being the lateral resistance in the doped region forming the BSF. Analytical expressions for the diﬀerent resistances can be found in [74, 89].

The contributions R4 −R7 are directly or indirectly influenced by the front side metallization. The contribution of the emitter sheet resistance Rsh to the total series resistance (resistance R4) depends on the finger distance df of the contact grid. By increasing the finger density (decreasingdf)R4 is reduced. Considering the contribution R6, the larger the cross-sectional area of the contact, the lower is the line resistance of the finger. As R6 additionally depends on the finger length, a high density of busbars reduces this contribution. For the lateral resistance of the busbar R7, the same considerations can be applied as for the fingers. In order to reduce the total series resistance, a grid design with a high density of wide fingers would be the best choice.

However, by raising the fraction of metalized wafer area, optical losses due to shadowing increase. The metal grid is designed to achieve a compromise between shading and series resistance losses. Therefore, the ﬁnger geometry is optimized to reach a high height-to-width aspect ratio.

R5 represents the contribution of the contact resistance to the total series resistance.

Like all resistances introduced in figure 1.9 this resistance is normed to a cell area of 1 cm² (unit: Ωcm²). To determineR5 from the specific contact resistanceρc of a contact finger, the area weightedρc is divided by the finger area fraction. A detailed discussion on the contact resistance including its measurement by means of the transfer length method (TLM) will be given in section 2.1.3.

1.6.5 Fill factor loss analysis for metallization induced losses

In order to analyze the influence of the different loss mechanisms on the fill factor, the different fill factors introduced in 1.5 can be compared in a fill factor loss analysis.

Comparing the ”real” fill factor FF with the pseudo fill factor pFF quantifies the fill factor losses due to series resistance losses RS, as pFF is independent of the series resistance. SincepFF still is influenced byj02 andRP, the difference betweenpFF and the ideal fill factor FFid gives the losses caused by recombination in the space charge region, e.g., as a result of in-diffusion of impurities or shunts due to deep metal spikes in contact with the base of the cell.

1.7 Metal-semiconductor contact

Screen-printed contacts generally feature direct metal-semiconductor interfaces, mainly at the silver crystals that grow into the silicon surface [90]. Therefore, understanding

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1.7 Metal-semiconductor contact

the current transport at these interfaces is very important when the current transport through the whole contact is considered. For the application in semiconductor devices, contacts with an ohmic, that means linear, current-voltage characteristic would be desirable. As will be presented in the next section, silicon-metal contacts do not show an ohmic behaviour according to this deﬁnition. For solar cell application, Schroder and Meier [91] therefore proposed another deﬁnition for an ohmic contact: ”the voltage drop across the ohmic contacts should be small compared to voltage drops across the active regions of the device or, stated another way, the ohmic contact should supply any current that the device requires in its normal mode of operation”.

1.7.1 Schottky contact

The rectifying nature of the metal-semiconductor contact was first discovered by Braun in 1874 [92]. Schottky provided the first explanation for the asymmetric behaviour of the current transport over metal-semiconductor interfaces and gave the metal-semiconductor contact its name ”Schottky contact” [93–95]. According to Schottky, when a metal and a semiconductor are brought in direct contact, the different Fermi levels in the solids equal in thermal equilibrium and a band bending occurs. The band bending depends on the work functions of metal Φ_M = q·φM and semiconductor Φ_S = q·φS, that are defined as the energy difference between vacuum level and Fermi level, with the corresponding potentials φM and φS. Depending on the relation of both potentials, a rectifying (n-type φM > φS, p-type φM < φS), neutral (φM = φS) or accumulation contact (n-type φM < φS, p-type φM > φS) that shows an ohmic current-voltage characteristic, forms. As a result of charge carrier movement, a depletion layer of the width Wb builds up in the semiconductor. At rectifying contacts a potential barrier called

”Schottky barrier” is formed. The rectifying case is shown for both, n- and p-type semiconductors, in ﬁgure 1.10 before contact and after the alignment of the Fermi levels, after the solids were brought in contact. Additionally, the Schottky theory presumes a strong inﬂuence of the metal work function on the barrier heightφB:

n-type: φBn =φM −χS (1.13)

p-type: φBp = Eg

q −(φM −χS). (1.14)

qχS is the semiconductor electron affinity, that is defined as the difference between conduction band edge and vacuum level.

To obtain an ohmic behaviour an accumulation contact would be desirable. According to Schottky, this would be possible by simply choosing a metal with an appropriate work function. Also, the height of the barrier could be adapted in this way in the

Contact Formation of Ag/Al Screen-Printing Pastes to Heavily B-Doped c-Si